Devices of this type are commonly used in order to control oscillation frequencies of frequency synthesizers, particular in radio signal receiver devices such as televisions, decoder boxes or cellular telephones, in which devices the frequency conversions are carried out by mixing signals to be converted with output signals of these synthesizers. A device for comparison, which is designed to control a frequency synthesizer, is known in particular from patent U.S. Pat. No. 5,216,374. As the first and second input signal, this device is designed to receive an output signal of an oscillator, and a reference signal. By means of a microcontroller, the known device calculates the frequency difference which exists between the input signals, and generates a digital control signal, which comprises a succession of pulses, which have width which is determined by the result of the said calculation. The digital control signal is then converted into an analogue voltage, and is applied to a control input of the oscillator, in order to adjust the frequency of its output signal. The known device thus uses digital circuitry which has a very complex structure, in order to produce a quantitative measurement of the frequency difference between the input signals, and to translate the result of this measurement into modulation of the width of the pulses of the control signal. Circuitry of this type has major design problems, owing in particular to the lengthy response times caused by its complexity. In addition, the known structure is difficult to produce in the form of a single integrated circuit, which makes it costly to produce, and difficult to use.
The object of the present invention is to eliminate these disadvantages, by providing a device for comparison which is provided with a very simple structure, which is easy to integrate, and the response time of which is very low.